1. Field of the Invention
The present invention relates to an image capturing apparatus including an image sensor.
2. Description of the Related Art
In an image capturing apparatus including an image sensor, as the exposure time prolongs and the temperature of the image sensor rises, dark current components increase in the image sensor to generate defective pixels, resulting in flaws, dark shading, and the like, which are superimposed as fixed pattern noise components on an output image. Such noise components become factors that cause deterioration in image quality. In general, it is possible to remove these fixed pattern noise components by performing the black subtraction processing of subtracting a black image (light-shielding image) captured under the same conditions as those for an actual exposure image from the actual exposure image.
According to Japanese Patent Laid-Open No. 1-39171, a dark time output signal corresponding to fixed pattern noise which an image sensor has is recorded in a memory in advance. When an object is shot, the above dark time output signal is read out from the memory and subtracted from the captured image, thereby performing fixed pattern noise correction.
Consider a case in which as shown in FIG. 12, stars moving in the night sky are continuously shot, and obtained actual exposure images are composed to generate a star trail image depicting star trails in the starlit sky in one picture. Since the night sky and stars as objects are low in illuminance at this time, it is necessary to set a sufficiently long actual exposure time (about several min to several ten min) in the image capturing apparatus. In addition, in order to compose actual exposure images as star trail images, it is necessary to continuously shoot the stars and compose images of the stars, which move from moment to moment, for a long period of time. Stars in the night sky move around the North Star through 15° per hour. Although it depends on the shooting direction and the lengths of trails to be shot, in order to record a shot picture as a star trail image, it is generally necessary to perform continuous shooting for several ten min to several hrs.
If, however, a long exposure time is set in the above manner, fixed pattern noise, such as flaws and dark shading, tends to occur. In addition, shooting for a long period of time will raise the temperature of the image sensor every second, and hence fixed pattern noise tends to occur accordingly. For this reason, in order to obtain a high-quality image as a star trail image like that described above, it is indispensable to perform fixed pattern noise correction.
It is normally preferable to remove fixed pattern noise by capturing black images before and after the shooting time of an actual exposure image, that is, under a condition in which the temperature of the image sensor hardly changes, and performing black subtraction processing for each actual exposure image. However, this processing makes it impossible to record the movement of stars during shooting for black images, and hence the star trail image obtained by composition becomes discrete, as shown in FIG. 13.